Fixed religious pressure bug, generic missionaries, and faith unit setup.

Fixed a bug that caused buildings which affect religious pressure to
multiply out of control. (Hello, 7000 pressure) Due to integer
multiplication.

The Civ religion AI is no longer hardwired to buy UNIT_MISSIONARY, it
should now also buy unique units that replace the missionary.

Faith units (missionary, prophet) previously required explicit setup in
CvCity.cpp. Any time a new unit is created (in initUnit), it now sets
itself up as a faith unit if all of the required information is present
for their unit type.